Rigging question about Spline IK and having all controls on one armature
It's possible, here is a flow chart of the dependencies:




You want all your bones in your Control rig (deform and non-deform), except your Spline IK bones.
The control rig will control Curve vertices with Hook modifiers.
The Curve controls IK Splines in deform rig. You want to copy all the deform bones from Control rig to Deform rig and copy transforms from Control.
The Deform rig now should have all the bones to deform character. Bind the mesh to this armature.
Parent everything to Control rig. You can hide the Curve and Deform rig from view and render.